Method for treatment of spent liquor

ABSTRACT

Method for treatment of spent liquor at a pulp mill, in which method at least a part of the spent liquor flow ( 10 ) arriving from the evaporation plant is taken to a pyrolysis reactor ( 1 ), wherein it is pyrolysed at a temperature of 300-800° C. in order to separate evaporable compounds ( 12 ) from coke ( 11 ) remaining in a solid state. The pyrolysis products ( 11 ), which are gases or liquids, may be used as fuel or they may be processed further. The coke ( 11 ) resulting from the pyrolysis is burnt in a soda recovery boiler ( 3 ) or in a gasification reactor ( 2 ) to regenerate cooking chemicals. The method is suitable for recovery of chemicals and energy both in sulphate and sulphite processes and also in cooking methods based on organic solvents.

The invention concerns a method for treatment of spent liquor from apulp mill in order to recover the chemicals and energy contained in theliquor. Spent liquor in this context means black liquor and such spentliquors resulting from sulphite cooking of different kinds as well asfrom other pulp cooking processes, which contain cooking chemicals aswell as organic substances dissolved from deligni-fled material.

In the pulp process, the fibrous raw material is cooked in a cookingchemical solution, which in the sulphate process contains sodiumsulphide and sodium hydroxide and in the sulphite process containssulphite solutions of different kinds. During cooking organic compoundswill dissolve from the wood material, and the most important of these isthe lignin binding the wood fibres to each other. After cooking, thefibres are separated from the spent liquor formed by cooking chemicalsand by the substances dissolved from the wood. In the sulphate processthis liquor is called black liquor, whereas in sulphite methods its moregeneral name is spent liquor. The dilute spent liquor existing afterwashing is evaporated to a dry matter content, which may be even 70-85%depending on the mill. Various cooking methods for separation of thefibres and based on organic solvents have also been presented. Thesediffer from the sulphate and sulphite processes as regards theircirculation of chemicals, among other things. To date the cookingmethods based on organic solvents have not achieved a significantposition in competition with the sulphate and sulphite methods, whichare more efficient when using modem technology.

After the evaporation plant, the spent liquor is processed by burning itin controlled conditions in a spent liquor boiler, which is usually asoda recovery boiler when using sodium-based cooking solutions. Theprimary task of the soda recovery boiler is to bring about favourableconditions for collecting in such a form the inorganic chemicalscontained in the spent liquor, that after regeneration they can onceagain be used in the cooking process. Another important task of the sodarecovery boiler is to recover the chemical energy contained in theorganic substance dissolved from the wood, which takes place as a normalsteam boiler process. As the organic substance burns, heat is releasedfrom it and the heat is used for producing high-pressure steam for theproduction of electricity and low-pressure steam for process use. Nosoda recovery boiler is needed in connection with cooking based onorganic solvents, but the circulation of chemicals takes place bydistilling or by some other chemical method. The substance containinglignin, from which the cooking chemicals have been separated, can beburnt, for example, in an ordinary fluidised-bed boiler or in some otherburning equipment.

The soda recovery boiler technology is a very conservative one. Theburning device in question is one resembling the steam boiler with astructure and operation that have mainly remained the same over decades.Improving the reliability and increasing the capacity while keeping theold principles of operation have been important aspects in thedevelopment. The soda recovery boiler is usually the biggest and mostexpensive component in the pulp mill and its investment costs areapproximately 15-25% of the total price of the mill. Since thecomposition of the spent liquor burnt in the soda recovery boilerentails problems to do with material technology, the values of steamproduced in the soda recovery boiler are low compared with conventionalpower boilers, which results in a poor power-to-heat ratio from theviewpoint of electricity production.

Alternative solutions have been presented for replacing the sodarecovery boiler, and of these the gasification of black liquor has comeclosest to commercial implementation. Patents FI 82494 and FI 91290describe examples of methods for recovering chemicals and energy basedon gasification of black liquor.

In FI Patent 82494, the black liquor is gasified in a pressurisedgasification reactor at a temperature of 700-1300° C. using air oroxygen as the gasification medium, whereby the organic substance of theblack liquor is converted entirely into gases.

The inorganic chemicals form a smelt consisting mainly of sodiumcarbonate and sodium sulphide. The heat needed by the reactions isproduced by using oxygen at the early gasification reactor stage to burnthe hydrogen and carbon monoxide obtained in the gasification. The gasis cooled, washed and used as fuel to generate steam and, ifeconomically profitable, to produce electric energy.

In FI Patent 91290, the black liquor is gasified with the aid of air ata temperature of 800-1200° C., whereby the inorganic compounds arerecovered in the melt phase as compounds that can be used in the cookingprocess and as energy of the organic compounds of the black liquor,which energy is mainly bound to the chemical compounds of the gas phase.The gases obtained in gasification and containing sodium compounds areconducted into a particle cooler and into a filter, from which thesodium dust is returned to the gasification device. The clean gas istaken to the gas turbine.

In spite of the great expectations on commercialisation of gasification,practice has shown that the energy efficiency of the gasificationprocess is poorer than that of traditional soda recovery boilers, atleast to date. Extra losses also relate to the conversion of energy. Theproduct gas formed by the mixture of combustible and non-combustiblegases has a relatively low thermal value. In addition, it is expensiveto clean the product gas, and the usability of gasification plants israther poor at the present time.

The present invention aims at a new manner of treating black liquor orother spent liquor of the pulp process in order to achieve the desiredfinal result in an economically more sensible way than has been achievedwith the traditional soda recovery boiler technology. Hereby the spentliquor is brought into such a form that it is possible to use it formaking valuable biologically-based fuels and other upgraded products.

Another objective is to allow utilisation of the chemical energy contentof the spent liquor in such a way that the share of electricityproduction can be increased in comparison with the traditionalsolutions.

An additional objective of the invention is a solution, which may beused when required to add to the insufficient capacity of the sodarecovery boiler or to replace the soda recovery boiler altogether.

In order to achieve the objectives presented above and those emerginghereinafter, the solution according to the invention is characterised bythe features presented in the characterising part of the independentclaim 1, 2, 3 or 4.

Claims 1, 2, 3 and 4 describe four ways of treating black liquor orother spent liquor, each one having in common the use of a pyrolysisprocess as one sub-step in the recovery of chemicals and energy. Thepyrolysis is carried out as a separate unit process, that is essentiallydifferent from the gasification, which is also applied as a furthertreatment step in two solutions according to the invention.

Pyrolysis in this context means a thermo-chemical process, wherein theheat introduced into the process separates the evaporable componentsfrom the treated solid or liquid organic substance. Chemical reactionstake place primarily only as internal reactions of the treated substanceand/or as reactions between the released gases and the treated substanceand/or as reactions between gases released from the treated substance.External components, such as gases leaked into the reactor vessel, willcause secondary reactions of minor significance only. In this case,distilling can be regarded as a special case of pyrolysis.

Gasification means turning the starting material into a gaseous state ina chemical process with the aid of heat and an external gaseouscomponent or components aside from the starting material. The mostgenerally used gasification components are air, O₂, CO₂ and H₂O. Asregards black liquor a method is also known, which is based on thereaction between Na₂CO₃ and H₂S and which is described in thepublication Magiiussolz, Hais, “Power and Chemicals—New Possibilitiesfor the Kraft Recovery Process”, Proceedings of the 1998 InternationalChemical Recovery Conference, Tampa, Fla., 1998, p. 981-982. This methodtoo is suitable for use for gasification of coke resulting frompyrolysis of spent liquor and for regeneration of salts in the coke inthe method according to the invention. No gasification componentscontaining free or bound oxygen are used in this method.

Primarily the gasification thus takes place with the aid of external gascomponents, in which respect the gasification is chemically differentfrom pyrolysis, which is chemical decomposition of the material broughtabout merely by external heat. As carried out in a gasification reactor,the gasification usually also contains a pyrolysis-like sub-process, butthe final products of this process are processed and admixed with theproducts resulting from the other chemical sub-processes of thegasification. For this reason, the product gases of the gasificationreactor are different from the product gases of the pyrolysis reactor.

In the method according to the invention, at least a part of the spentliquor arriving from the evaporation plant and concentrated to adry-matter content of 70-85% is pyrolysed at a temperature of 300-800°C. in order to separate the volatile compounds contained in the spentliquor from the coke remaining in a solid state. The above-mentioneddry-matter content range may be regarded as a guideline, and in somecases it may be more advantageous to use some other dry-matter content.Besides the spent liquor, only heat may be supplied into the pyrolysisreactor, but no gas containing oxygen. The pyrolysis is carried out insuch conditions where the sulphur and sodium contents of the blackliquor will mainly remain in the coke. The distillate formed by thepyrolysis products is recovered, purified and used in suitableapplications at the mill and/or it is processed and/or sold outside. Thecoke is taken to combustion either in a soda recovery boiler or in agasification reactor. In either case the combustion is carried out inreducing conditions in such a way that the leftovers of cookingchemicals contained in the coke will be reduced into a form required forregeneration of the chemicals, that is, mainly into sodium sulphide andsodium carbonate. Alternatively, the gasification and reduction may becarried out by using hydrogen sulphide, whereby the sodium carbonatewill react with the hydrogen sulphide forming sodium sulphide. Inaddition, gases will result (H₂, CO, H₂O and CO₂). The heat needed forgasification may be produced e.g. by a gas or oil burner or byelectricity.

In a first embodiment of the method, only a part of the spent liquorflow arriving from the evaporation plant is pyrolysed and a part istaken directly to the soda recovery boiler. The coke resulting frompyrolysis is burnt in the soda recovery boiler.

A second embodiment of the method also takes a part from the spentliquor flow arriving from the evaporation plant to the soda recoveryboiler while a part is pyrolysed. The coke resulting from pyrolysis istaken to a gasification reactor, wherein a reduction of sulphurcompounds and carbonates is also carried out, besides the gasificationof coke.

In a third embodiment of the invention, the entire spent liquor flowarriving from the evaporation plant is taken to pyrolysis and all cokeresulting from the pyrolysis is gasified, whereby there is no need forany traditional soda recovery boiler. In a fourth embodiment of theinvention, the entire spent liquor flow arriving from the evaporationplant is taken to pyrolysis. The coke resulting from pyrolysis is burntin a fluidised-bed boiler or in some other burning equipment. Herebyburning may take place e.g. in the mill's bark boiler.

The first three embodiments of the invention are very suitable inconnection with sulphate and sulphite processes, and the fourth e.g. inconnection with cooldng carried out with an organic solvent (ethanol,formic acid, etc.).

The pyrolysis is carried out in a separate pyrolysis reactor either as acontinuous process or as a batch process. The continuous process allowsa higher treatment capacity per volume unit of the reactor. Advantagesof the batch process are easily implemented fractionation of theproducts, purity of products as the ash remains in the coke, and a highthermal value of the product gas in energy production. The pyrolysisproducts may be gases, such as carbon monoxide, hydrocarbons and wateror pyrolysis oils or both. The products can be processed further.

Pyrolysis of the spent liquor in a continuous process is carried outwithin an approximate temperature range of 300-800° C., wherein thetemperature and other process conditions are chosen depending on thekind of desired final products. The lower limit of the temperature rangeis an experimentally determined temperature, at which all volatileorganic components are made to evaporate from the spent liquor, whilethe upper limit is a temperature, at which sodium compounds begin movingover into the product gas to a significant extent. The optimumtemperature for pyrolysis is between 550 and 650° C. Hereby the releaseof sulphur is less than at lower temperatures and, on the other hand,alkali metals will not yet be released into the pyrolysis product. Theinitial temperature for the batch process is determined according to thetemperature of the spent liquor supplied to the reactor, whereby it mayremain considerably below 300° C. The final temperature and the heatingspeed may be chosen according to the desired products. Other processvariables affecting the quality of the final pyrolysis productsare—besides the quality of the spent liquor—for example, the residencetime, the heating speed and the pressure.

The solid final product remaining after the pyrolysis of spent liquor,that is, the coke, which contains a major part of the inorganicchemicals of the liquor, is either burnt in a soda recovery boiler or itis gasified in a gasification reactor. If the liquor is black liquorand/or sodium-based spent sulphite liquor, the coke taken togasification must contain free carbon, so that reduction of Na₂SO₄ toNa₂S is possible. The gasification is carried out within a temperaturerange of 1000-1400° C., whereby it is possible to guarantee asufficiently high temperature for carrying out reductive reactions. Thegasification may be carried out at atmospheric pressure or aspressurised gasification, and oxygen, carbon dioxide, water vapour ortheir mixture may be used as the gasifying component. When treatingblack liquor or any other sodium-based spent liquor, chemicalgasification with hydrogen sulphide may also be used.

From the viewpoint of a sulphate and sulphite mill, the main products ofthe presented process are the recovered cooking chemicals, which aretaken to the normal circulation of chemicals at the mill. The pyrolysisproducts may be used as fuel at the mill or they may be processedfurther e.g. into methanol, ethanol, etc. The gases brought about in thegasification may be burnt in a boiler, in a gas power engine, in a paperimpingement dryer, in a lime kiln or in other applications of a similartype.

The process is flexible, allowing various parallel and seriesconnections. With the invention it is possible to increase the spentliquor treatment capacity at the mill and to postpone purchasing of theexpensive soda recovery boiler in situations where the boiler capacityis a factor limiting production. It makes it possible to raise the valueof black liquor or other spent liquor with the aid of further processingand a higher power-to-heat ratio. The power-to-heat ratio of electricityproduction may be increased in comparison with the traditional sodarecovery boiler solution. The emissions of carbon dioxide from the pulpmill are reduced, because the use of gases and pyrolysis products at themill makes it possible to stop using fossil fuels or at least to reducetheir quantity.

In the following, the invention will be described in greater detail withreference to the figures shown in the appended drawings, but theintention is not to limit the invention strictly to the details shown inthe figures.

FIG. 1 is a simplified view of a first embodiment of the invention ofthe spent liquor treatment process according to the invention, whereinonly a part of the spent liquor is pyrolysed and the resulting coke isburnt in a soda recovery boiler.

FIG. 2 is a simplified view of a second embodiment of the invention,wherein only a part of the spent liquor is pyrolysed and the resultingcoke is gasified in a gasification reactor.

FIG. 3 is a simplified view of a third embodiment of the invention,wherein the entire flow of spent liquor is treated in pyrolysis andgasification reactors.

FIG. 4 is a simplified view of a fourth embodiment of the invention,wherein the entire flow of spent liquor is pyrolysed and the coke isburnt in a fluidised-bed boiler or in some other burning equipment.

FIG. 1 shows recovery of the chemicals of black liquor based on a sodarecovery boiler, wherein a part of the flow of black liquor 10 arrivingfrom an evaporation plant is taken directly to a soda recovery boiler 3,while a part is taken to a pyrolysis reactor 1, of which there may beone or more in parallel. The pyrolysis reactor 1 may be used for a batchprocess or for a continuous process.

The pyrolysis is carried out in a temperature range of 300-800° C.,whereby only heat is supplied into the reactor 1, and the heat makes theeasily evaporating compounds in the black liquor evaporate and/or turninto gases. In batch-type pyrolysis, increasing of the temperaturebegins from the temperature of the spent liquor arriving from theevaporation plant, and the temperature is chosen according to thedesired pyrolysis products. No oxygen or other gas is supplied to thereactor 1. The pyrolysis products 12 that have moved into the gas phaseare taken away from the reactor 1 into further treatment steps, whichmay be washing, condensing of condensable products etc. Depending on thetemperature, duration, pressure and other such factors of the pyrolysis,the final pyrolysis products 12 may be gases or liquids. The combustiblegases and/or pyrolysis oil produced by pyrolysing the spent liquor areused in suitable applications at the mill and/or they are processedand/or they are sold outside.

Another final product of pyrolysis is coke 11 in a solid state and alsocontaining, besides carbon, inorganic chemicals remaining from thecooking chemicals. In the example shown in FIG. 1, the coke 11 is talkeninto the soda recovery boiler 3 for burning, whereby in connection withthe burning reduction of sulphur to sulphide also takes place, which isnecessary for regeneration of the cooking chemicals. The coke 11 may besupplied into the soda recovery boiler 3 either admixed with the spentliquor 10 or as a separate supply. From the lower part of the sodarecovery boiler 3 smelt 13 is discharged, which when the liquor is blackliquor is dissolved in a manner known as such in water or in weak whiteliquor to form green liquor.

With the aid of pyrolysis it is possible to produce pyrolysis productsof a good quality and these may be used in many applications both at themill and outside the mill. Pyrolysis gases may be used as supportingfuel in a heat recovery boiler or as fuel in a lime kiln. They may beused for additional superheating of the soda recovery boiler or inimpingement drying in a papermaking machine. They are suitable as energysources when producing electricity by using a gas turbine. Pyrolysisoils are suitable not only as fuel but also as raw material for variousfurther processing products, such as methanol and ethanol.

FIG. 2 shows a solution, which is especially suitable for situations,where the capacity of the soda recovery boiler is a limitation to anincrease of the pulp mill's production. A part of the spent liquor flow10 arriving from the evaporation plant is taken directly to the sodarecovery boiler 3 and a part is taken to a pyrolysis reactor 1, whereinthe evaporable compounds contained in the spent liquor are separatedfrom the coke 11 remaining in a solid state. Differing from the solutionshown in FIG. 1, the coke 11 is not taken to the soda recovery boiler 3,but it is taken to a gasification reactor 2, wherein chemical reductionof salts also takes place.

In the gasification reactor 2, heat and a gasifying component 16 areused to turn the coke 11 into product gas 14 and smelt 15, which iscombined with the smelt 13 arriving from the soda recovery boiler 3. Inorder to bring about reduction of sulphur, a higher temperature must beused in the gasification than in pyrolysis. In gasification the usualtemperature range is 1000-1400° C., and at least a part of the requiredheat is generated by burning coke and gases formed in the gasification.

The product gas 12 of the pyrolysis reactor 1 is separated, purified andused in suitable applications at the mill and/or it is processed and/orsold outside. Product gas 14 is also obtained in the gasificationreactor 2, and this gas is purified and used at the mill in a suitableapplication. Pyrolysis gases are of a better quality than the gasesresulting from gasification, since they contain hydrogen of the fuel andthey have relatively more unburnt fractions (hydrocarbons etc.) than thegasification gases. For this reason, they are very suitable for furtherprocessing.

FIG. 3 shows an alternative solution for recovery of chemicals andenergy of the spent liquor, wherein the traditional soda recovery boileris entirely replaced by a pyrolysis reactor 1, which is followed by agasification reactor 2. Several pyrolysis reactors 1 and gasificationreactors 2 may be built in parallel, whereby the spent liquor treatmentcapacity can be sufficient for the entire spent liquor flow 10. Theprocess conditions in the different reactors may also be varied in orderto obtain desired products of several kinds.

The gas 14 obtained from the gasification reactor 2 and containingcombustible compounds must usually be purified in order to separatesolid particles. After the purification, the product gas of thegasification can be taken, for example, to a combustion boiler, a gaspower engine or a gas turbine. The product gas of gasification may beused to replace natural gas both in energy production and in many piecesof process equipment in the pulp or paper mill, such as the lime kiln orthe impingement dryer.

The gases produced by the pyrolysis reactor can also be used as anenergy source, mainly for the same applications as gases produced bygasification. Advantages of pyrolysis gases are their higher thermalvalue and higher degree of purity in comparison with gases produced bygasification. In addition, the pyrolysis process allows production ofpyrolysis products in a liquid state.

FIG. 4 shows a process, which is especially suitable for treatment ofspent liquors resulting in cooking processes based on organic solvents.The concentrated spent liquor 10 is taken to the pyrolysis reactor 1,wherein it is distilled with the aid of heat to obtain a separatesolvent 12, which may then be used again in cooking. The coke 11remaining in the pyrolysis is burnt, for example, in a fluidised-bedboiler or other burning equipment 4 in order to recover the energy boundtherein. Combustion air 16 is supplied to the combustion boiler 4 andthe combustion produces flue gases 17 and ash 18.

In the following claims are presented defining the inventive idea,within which the details of the invention may vary and differ from theabove, which was presented by way of example.

1. Method for treatment of spent liquor at a pulp mill, especially for treatment of black liquor, in order to recover its contents of chemicals and energy, wherein a spent liquor flow (10) arriving from the evaporation plant is taken to a pyrolysis reactor (1), wherein it is pyrolysed at a temperature of 300-800° C. in the absence of an external gas component in order to separate evaporable compounds (12) from the coke (11) remaining in a solid state, whereupon the evaporable compounds (12) are recovered and the coke (11) is taken to a gasification reactor (3) for gasification, which gasification is implemented under such conditions that the sulphur compounds contained in the coke (11) and deriving from the cooking chemicals are reduced to sodium sulphide.
 2. Method according to claim 1, wherein only a part of the spent liquor flow (10) arriving from the evaporation plant is taken to the pyrolysis reactor (1), whereas a second part of the spent liquor flow (10) is taken to a soda recovery boiler (3) where it is burnt in order to recover its contents of chemicals and energy.
 3. Method according to claim 1, wherein the evaporable compounds (12) separated from the spent liquor in the pyrolysis reactor (1) are used at the mill as fuel in part or entirely.
 4. Method according to claim 1, wherein the evaporable compounds (12) separated from the spent liquor in the pyrolysis reactor (1) are processed further.
 5. Method according to claim 1, wherein the product gases (14) resulting from the gasification are used at the mill as fuel in part or entirely.
 6. Method for treatment of spent liquor at a pulp mill in which cooking is carried out with an organic solvent in order to recover its contents of chemicals and energy, wherein the spent liquor flow (10) arriving from the evaporation plant is taken to a pyrolysis reactor (1), wherein it is pyrolysed at a temperature of 300-800° C. in the absence of an external gas component in order to separate evaporable compounds (12) from the coke (11) remaining in a solid state, whereupon the evaporable compounds are recovered and used at the mill as process chemicals in part or entirely, and the coke is taken to a fluidised-bed boiler or some other combustion equipment (4) for burning in order to recover the energy content of the coke.
 7. Method according to claim 1 wherein the pyrolysis reactor (1) is for a batch process, whereby increasing of the temperature may begin from the temperature of the spent liquor arriving in the reactor, while the final temperature is chosen according to the desired final products.
 8. Method according to claim 1, wherein the pyrolysis reactor (1) is for a continuous process.
 9. Method according to claim 1, wherein the pyrolysis is carried out in such process conditions (temperature, pressure, residence time, heating speed, etc.), wherein the evaporable compounds (12) mainly consist of non-condensing gases.
 10. Method according to claim 1, wherein the pyrolysis is carried out in such process conditions (temperature, pressure, residence time, heating speed, etc.), wherein the evaporable compounds (12) mainly consist of pyrolysis oil. 